With the fast growth of portable electronics and equipment, the demand for power converters that are lower in cost, higher in efficiency, and smaller in size is increasing. A high switching frequency is often the result of using smaller components to meet these demands. Synchronizing multiple power converters operating at increasingly higher switching frequencies presents challenges, especially in integrated circuit applications having multiple power converters.
A conventional DC-DC converter capable of reaching limited switching frequency operation is shown in FIG. 1 (prior art). In this circuit, DC-DC converter IC 10 regulates an output voltage via a resistive feedback divider 11 made up of resistors 12 and 13 between the output voltage and a ground node GND. A center tap of resistive divider 11 is connected to a feedback amplifier 14 that amplifies the difference between a feedback signal 15 and a reference voltage from a voltage reference circuit 16. The signal at the output of feedback amplifier 14 is then converted into a duty-cycle signal 17 by a controller 18 that bases the switching frequency on an oscillator 19. Duty-cycle signal 17 then drives a main switch 20 and a synchronous rectifier 21, generating a voltage pulse waveform at an inductor switching node (LX) 22. An inductor 23 and an output capacitor 24 act as a low pass filter to filter out the voltage pulse waveform to supply an output voltage onto the output voltage node 25. Because of the negative feedback effect, the output voltage is regulated so that the feedback voltage is essentially equal to the reference voltage. Unfortunately, delays in feedback amplifier 14 and controller 18 both act to limit the switching frequency of converter IC 10.
FIG. 2 (prior art) shows an example of another prior art DC-DC converter IC 30 capable of higher frequency operation than the circuit shown in FIG. 1. In this architecture, a feedback resistor 31 and a second resistor 32 are connected in series between an inductor switching node (LX) 33 and a feedback node (FB) 34 to provide a DC feedback path. In addition, a feed-forward capacitor 35 is connected in parallel with resistor 32 between feedback node (FB) 34 and a hysteretic feedback comparator 36 to provide a high frequency feedback path. Feedback node (FB) 34 is also connected to an output voltage node 37. An inductor 38 is connected between inductor switching node (LX) 33 and output voltage node 37. Hysteretic feedback comparator 36 compares a feedback voltage 39 to a reference voltage from a voltage reference circuit 40 and generates a signal to control a main switch 41 and a synchronous rectifier 42 via a controller 43. In this circuit, the switching frequency is a function of feedback resistor 31, feed-forward capacitor 35, an hysteresis voltage output by feedback comparator 36, the input voltage 44, and the output on output voltage node 37. Converter IC 30 can achieve higher frequency operation than converter IC 10 because it is easier to stabilize, the output voltage variation is forward-fed into feedback node (FB) 34, and there is less delay in the loop. Converter IC 30 is a free-running, hysteretic DC-DC converter that is not synchronized with any external clock.
Power converters such as converter IC 30 are often referred to as hysteretic converters because they employ a hysteretic comparator. Power converters such as converter IC 10 are often referred to as clock-based because they include a clock, such as oscillator 19 illustrated in FIG. 1.
In many applications, multiple switching power converters are synchronized to minimize interference and noise. However, it is often difficult to synchronize multiple power converters operating at high switching frequencies, such as power converters that are not clock-based. As hand-held and/or portable devices continue to decrease in physical size, there is a need to reduce inductor and capacitor component sizes by increasing the switching frequency of the converters in power management unit (PMU) integrated circuits. In these power management applications, multiple switching power converters within a single integrated circuit require both high switching frequency and synchronized switching. In view of the foregoing, there is a need for a solution to synchronize multiple switching power converters operating at high switching frequencies in an integrated circuit.